1. Field of the Invention
The present invention relates to a method of forming a metal-semiconductor contact. More particularly, the invention relates to a method of contacting a semiconductor, such as silicon, which has been rough-cut using, for instance, a water-cooled diamond saw and a metal, such as tungsten, using a novel solder layer. Specifically, the invention involves the contact and bonding of a rough cut silicon to tungsten using a solder comprising palladium and silver.
2. Background of the Invention
The problems related to the permanent bonding of a metal electrode and a semiconductor material using a solder composition have been known for some time. For instance, H. Onodera et al., disclose in U.S. Pat. No. 4,110,783, the manufacture of a semiconductor device comprising a silicon body having an exposed surface of N-type conductivity layer and a substrate bonded to the exposed surface by means of a layer of a novel solder material. The solder material is an alloy consisting essentially of 2-12% by weight of at least one element of Group V of the Periodic Table, preferably antimony, and 0.01-5% by weight of at least one of the rare earth elements, for example Misch metal (containing lanthanum and cerium) and aluminum being the balance on the basis of total weight of the solder material. An increase in forward voltage drop (FVD) of the device in which a conventional aluminum solder is used is prevented by the use of the new solder materials.
In U.S. Pat. No. 4,476,375, Ogawa discloses a method for cutting an electrical conductive layer having a heat-absorbing film deposited thereon by irradiating an energy beam on a portion of the electrical conductive layer intended to be cut. The energy beam is irradiated on the film, whereby the portion of the electrical conductive layer melts and alloys with the heat absorbing film due to the energy absorbed and transferred by the heat absorbing film. The selective cutting electrical conduction layers using a laser and melting of a metallic circuit using excess current are also disclosed.
A general discussion by G. Sistare, et al., on "Solders and Brazing Alloys" is found in the Kirk-Othmer: Encyclopedia of Chemical Technology, (3rd ed.), Vol. 21, p. 342-355, published in 1983 and also "Silver and Silver Alloys" on pages 1-15.
Contact formation on a semiconductor substrate is usually obtained by means of a deposition of a metal and sintering at high temperatures. Contact formation also occurs by means of alloying done during semiconductor device fabrication only after several involved steps are followed for preparation of the surface of the substrate in the contact area. These steps are performed in order to obtain a series of properties on the surface of the substrate in the contact area, including, for example:
1. A lack of or low density of crystal defects which degrade the electrical properties of the contact and which are produced when the substrate is cut by a water cooled diamond saw at the surface where the contact is to be formed. PA0 2. The absence of a native oxide layer, which in all but the metal-insulator-semiconductor (MIS) devices is not desired in metal-semiconductor contacts, neither of ohmic nor rectifying types. PA0 3. A fairly flat surface of the substrate in the contact area in order to achieve a reproduceable current density distribution in the contact area. PA0 (a) contacting the surface of the semiconductor material in an inert atmosphere between about 800.degree. and 1300.degree. C. wherein said surface is rough cut and is not subsequently treated further, with a metal electrode wherein the surface of said metal electrode comprises a solder itself comprising between about 10 to 90 percent by weight of palladium and about 10 to 90 percent by weight of silver; PA0 (b) creating a solid contact; and PA0 (c) cooling the soldered contact in inert atmosphere to ambient temperature. PA0 (1) contacting the rough cut silicon surface in an inert atmosphere at between about 800.degree. and 1300.degree. C. with a tungsten electrode wherein the surface of the tungsten electrode comprises a solder surface layer, itself comprising between about 24.8 and 37.2 mg of the palladium and between about 43.2 and 64.8 mg of silver per square centimeter of the surface of the tungsten electrode; PA0 (2) creating a solid silicon-solder-tungsten contact; and PA0 (3) cooling the contact in inert atmosphere to ambient temperature.
It is common practice in the art to perform several process steps to obtain the above cited properties. The process steps in current technology include, for example, grinding, chemical polishing, chemical etching, reactive plasma etching or the like are expensive and time consuming. In the case of the use of chemical agents, such as highly toxic and extremely reactive nitric acid or hydrogen fluoride, special precautions are needed to protect the surroundings and the personnel from contact and contamination. These process steps are time consuming, and in some cases hazardous, and detract from the efficiency of manufacturing semiconductors.
It is therefore very valuable to decrease the number of process steps in the fabrication of a semiconductor device. It is also important to eliminate, if possible, the need to use hazardous agents in chemical polishing and etching. These objectives are accomplished in the present invention by elimination of all of the above cited steps for preparation of the surface of the substrate in the area where the alloyed contact will be formed.